Pyrolysis processes, such as steam cracking, are widely utilized for converting saturated hydrocarbons to higher-value products such as light olefins, e.g., ethylene, propylene and butenes. Conventional steam cracking utilizes a pyrolysis furnace that has two main sections: a convection section, and a radiant section. In the conventional pyrolysis furnace, the hydrocarbon feedstock enters the convection section of the furnace as a liquid (except for light feed stocks which enter as a vapor) wherein it is heated and vaporized by indirect contact with hot flue gas from the radiant section and optionally by direct contact with steam. The vaporized feedstock and steam mixture (if present) are then introduced through crossover piping into the radiant section where the cracking takes place. The resulting products comprising olefins leave the pyrolysis furnace for further downstream processing.
Although pyrolysis principally involves heating the hydrocarbon feedstock sufficiently to cause thermal decomposition of the larger molecules, the process also produces molecules that tend to combine to form high molecular weight materials, the heaviest of which are steam cracked gas oil (“SCGO”) and steam cracked tar (“SCT”). Not only are SCGO and SCT among the least valuable products obtained from the effluent of a pyrolysis furnace, feedstocks containing higher boiling materials (“heavy feeds”) generally tend to produce greater quantities of SCGO and SCT. Thus, as the refining industry is required to process more heavy feeds, there is a growing need to upgrade these heavy pyrolysis products.
For example, SCGO is a highly aromatic, hydrocarbon fraction boiling in the range 350 to 650° F. (177 to 343° C.), normally 400 to 550° F.(204 to 288° C.), and composed mainly of C10 to C17 hydrocarbons. The combination of its high aromaticity and its desirable boiling point distribution make SCGO a potentially attractive solvent, especially in the upgrading of SCT. However, SCGO typically has a high olefin content, with 3.0 wt % of the hydrogen atoms being olefinic, as measured by 1H NMR peak integration. In addition, SCGO typically has a high sulfur content, generally in excess of 0.5% by weight. Both of these properties currently prevent SCGO from being a high value product. Olefins are unstable and have a tendency to polymerize at higher temperatures. This prevents the use of SCGO as a solvent for SCT hydroprocessing due to increased problems with reactor fouling. In addition, its high sulfur content effectively prevents SCGO from being used as an additive for fuels.
There is therefore a need for a simple and effective method of upgrading SCGO by decreasing its olefin content and/or its sulfur content.